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For applying the optimum backwash method to activated carbon absorption process, this study had performed an efficiency test of backwash method and a test for determination of backwash period at the M water purification plant in Daegu metropolitan city. The minimum fluidization velocity was different according to kinds of carbon like spent carbon and reactivated carbon. Changing water position before backwashing was more efficient in backwashing than controlling backwash time. In the case of water position LL(a height of 60cm over the outer layer of activated carbon) before backwashing, the most efficient backwash method has turned out to be 10 min. of air wash and 18 min. of water wash. The turbidity of activated carbon filter outflow water and organic matter change have no big difference according to the days of seasonal operation after backwashing. As backwash period is very related to microbiological growth and is influenced by outflow water change, the study has found that it's desirable to operate in consideration of HPC(Heterotrophic plate counter) distribution of filtered outflow water, water quality, the condition of a filter basin and the years of activated carbon use.
Journal of Water and Environment Technology, Vol.1, No.2, 2003 A Study on the Optimum Backwashing Method applied to Activated Carbon Process in Waterworks Bok-Sil Ko, Ho-Souk Yoon, Sin-Jung Park, Min-Hye Yoon, Teak-Gyu Kwon, Sun-Koog Kwon and Jong-Woo Kim* Maegok Water Purification Plant, Water Quality Research Institute*, Daegu Metropolitan City Abstract For applying the optimum backwash method to activated carbon absorption process, this study had performed an efficiency test of backwash method and a test for determination of backwash period at the M water purification plant in Daegu metropolitan city The minimum fluidization velocity was different according to kinds of carbon like spent carbon and reactivated carbon Changing water position before backwashing was more efficient in backwashing than controlling backwash time In the case of water position LL(a height of 60cm over the outer layer of activated carbon) before backwashing, the most efficient backwash method has turned out to be 10 of air wash and 18 of water wash The turbidity of activated carbon filter outflow water and organic matter change have no big difference according to the days of seasonal operation after backwashing As backwash period is very related to microbiological growth and is influenced by outflow water change, the study has found that it's desirable to operate in consideration of HPC(Heterotrophic plate counter) distribution of filtered outflow water, water quality, the condition of a filter basin and the years of activated carbon use -Key Words: Optimum backwashing method, Minimum fluidization velocity, Heterotrophic plate counter, Activated carbon Ⅰ Introduction In advanced water purification, the absorption process of granular activated carbon removes, very efficiently, not only taste, smell, or color, but every kind of pollutants such as DBPs(Disinfection By-Products), BDOC(Biodegradable Dissolved Oxygen Carbon), SOCs(Synthetic Organic Chemicals), and VOCs(Volatile Organic Compounds) organic matter of a small amount in water1) Granular activated carbon has many angles and irregular shape, and can cause some problems So it may create mudball; may leak minute activated carbon and microorganism; its low specific gravity causes loss in backwashing Therefore, it requires proper management and careful operation2) Generally, determining the date of backwashing in sand filter basins is based on the head loss of a filter layer, the leakage turbidity of processed water, and filter duration But the quality of water flowing into the filter basin of granular activated carbon is mostly stable because it has passed - 189 - Journal of Water and Environment Technology, Vol.1, No.2, 2003 through sand filtration and later ozone processing; it has a little suspension and head loss doesn't increase greatly according to filter duration Therefore, it isn't enough to determine the date of backwashing only by the head loss of a activated carbon layer and the turbidity of processed water3) In filter process, backwashing makes suspension in filter medium dropped off and removed from filter medium by proper wash methods; can increase filter efficiency after sufficient washing, and improve productivity because of increase in filter duration and decrease in backwash frequency However, insufficient wash effect lessens filter duration, deteriorates the quality of filtered water because of leaked suspension, and causes other problems, which can have a direct influence on the quality of purified water Thus, with granular activated carbon absorption process of the M water purification plant in Daegu Metropolitan City as the subject of examination, the study has compared backwash efficiency according to backwash methods, and analyzed filtered outflow water according to operation time after backwashing in order to extract factors necessary for determining the optimum backwash period as a base for efficient management of advanced water purification facilities Ⅱ Experimental conditions and methods The study selected basins(2 reactivated, spent carbon basins, and virgin carbon basin) of granular activated carbon from 24 ones in the M water purification plant in Daegu; tried to find out the optimum condition by changing the time and method of seasonal backwash from October, 2001 to September, 2002 Specifications and operation conditions of Granular activated carbon 24 granular activated carbon contact basins consist of buildings each of which has stationary downward filter basins The rate of activated carbon and sand is 250:20(㎝); the under drainage system is strainer-type Backwashing uses both air wash and water wash; air wash velocity is 0.83 ㎥/min·㎡ and water wash velocity 0.4㎥/min·㎡(Table1) Table The present condition of granular activated carbon contact basin facilities Division Conditions The charge amount and indexes of activated carbon The method of current method 250m (8mⅹ12.5mⅹ2.5m), 24 basins Stationary downward current Empty bed contact time 10minutes LV and SV 15m/hr and 6l /hr Backwash conditions Air wash velocity 0.83m3/(min.·m2) Water wash velocity 0.40m3/(min.·m2) - 190 - Journal of Water and Environment Technology, Vol.1, No.2, 2003 The characteristics of a granular activated carbon In a granular activated carbon contact basin, virgin carbon is domestic activated carbon made from palm shell, reactivated carbon means activated carbon produced in the compound regenerative facilities, and spent carbon is activated carbon used for over years The specification of three carbons are expressed in Table Table The specification of activated carbons Spent* Reactivated** Virgin** (m2/g) 1,066 1,157 1,030 MB absorption (㎎/g) 100 190 150 Iodine value (㎎/g) 680 1,060 1,130 Items Specific surface area * Spent : examined in October, 2001 ** Reactivated, Virgin: examined in June, 2001 Experimental methods 1) An efficiency test of backwashing In order to examine backwash efficiency in a granular activated carbon contact basin, the study has measured the minimum fluidization velocity and backwash discharged-water turbidity of backwashing by changing backwash methods as in Table And through a test of the minimum fluidization, the study has measured head loss values, and regarded as the minimum fluidization velocity the time when their measurements are constant Table Backwash methods in a granular activated carbon contact basin Process Methods of air wash and 18 of water wash Backwash time change Water position backwashing change 12 of air wash and 20 of water wash before A height of 110㎝ over the outer layer of activated carbon(water position L) A height of 60㎝ over the outer layer of activated carbon(water position LL) 2) A test of determining the date of backwashing In order to determine the proper date of backwashing for a granular activated carbon contact basin, the study has divided seasons like this - spring(March to June), summer(July to September), autumn(October and November) and winter(December to February); at the beginning of every season, for 10 days the study just picked outflow water from basins(spent, reactivated and virgin carbon) every day and examined items like turbidity while operating and not backwashing them The analysis of filtered outflow water was based on the official test methods of water pollution4), Standard Methods5), and the Japanese waterworks test methods6; each-item analysis equipment and test methods are as follows: - 191 - Journal of Water and Environment Technology, Vol.1, No.2, 2003 (1) Turbidity On picking water, turbidity was measured by Turbidimeter(HACH 2100) (2) UV254 UV254 was analyzed at 254nm by UV/Vis spectrophotometer(JASCO V-560) (3) KMnO4 consumption KMnO4 consumption was experimented in accordance with the official test method of water quality (4) TOC(Total organic carbon) On picking water, TOC was measured by TOC analyzer(SHIMAZU 5000A) (5) THMFP(Trihalomethane formation potential) Until free residual chlorine became 1.0∼2.0㎎/l , chlorine was poured in; pH was controlled into 7±0.2 by phosphoric acid buffer solution; it was settled at 20±1℃ for 24±1 hours Then the remaining chlorine quantity was measured; the water was picked into 50㎖ vial Right after that, arsenious acid sodium and phosphoric acid(1+10) were added there and THMs were measured; the early THM values were deducted from their measurements and the remaining values were THMFP (Purge&Trap/HP5890 GC) (6) HPC(Heterotrophic plate counter) On picking water, a sample of 1㎖ was diluted step by step and put on R2A agar; it was cultured at 20±1℃ for days; HPC was measured (7) The quantity of germs attached to activated carbon A sample was picked by an activated carbon picker inserted, by less than 1m, into the filter layer of a granular activated carbon contact basin once every month; picked granular carbon of 50g was put into 100㎖ of sterilized and distilled water; while the water was stirred for min., the carbon was washed times and then dried naturally for about hours After that, 20㎖ of sterilized saline solution was poured to the dried activated carbon of 1g, and the carbon was processed ultrasonically(40㎑, 180W) for min.; 1㎖ of the sample was diluted step by step in R2A; was cultured for days at 20±1℃ The germ quantity attached to activated 7,8,9) carbon was expressed the number of germs per 1g Ⅲ Results and consideration The results of an efficiency experiment according to backwash methods The turbidity of discharged water from backwashing is used as one of the important factors evaluating backwash efficiency Generally, increase in water temperature needs raising backwash velocity10,11), but the backwash equipment of granular activated carbon in the M water purification plant is uncontrollable because the condition of air is fixed in 0.83㎥/min·㎡ and that of water in 0.4㎥/min·㎡ Also, the water-position regulator was divided into steps like LL(a height of 60cm over the outer layer of activated carbon), L(a height of 110cm over it), H(a height of 210cm over - 192 - Journal of Water and Environment Technology, Vol.1, No.2, 2003 it), and HH(a height of 300cm over it); until now, backwashing has been performed at L water position Accordingly, as a method for raising backwash velocity according to increase in water temperature, water position before backwashing will be controlled downward to LL and backwash effect be improved 1) The turbidity of discharged water according to backwash time and changing water position before backwashing Table shows the maximum turbidity of discharged water caused by change in backwash time The maximum turbidity of discharged water from backwashing by air for 12 and by water for 20 at the L water position before backwashing was 4.2∼17.3NTU and higher than that from backwashing by air for and by water for 20 It was 4.2NTU in spent carbon with days of backwash period, 17.3NTU in spent carbon with days, 7.6NTU in reactivated carbon with days, and 7.3NTU in reactivated carbon with 8days Table The turbidity of discharged water by change in backwash time The maximum turbidity of discharged water (NTU) Increase and 8-min, of air wash 12-min of air and 18-min of wash and 20-min decrease water wash of water wash Division 4days of back wash 6days of back wash 8days of back wash Spent 27.3 31.5 + 4.2 Spent 31.2 48.5 +17.3 Reactivated 45.6 53.2 + 7.6 Reactivated 53.5 60.8 + 7.3 Remarks *Water temp at the time of measurement : 4∼9℃ Table shows the turbidity of discharged water caused by change in water position before backwashing The maximum turbidity of discharged water from backwashing by air for 12 and by water for 20 was 15.6∼40.2NTU and higher at LL water position than at L water position And it was 15.6NTU in spent carbon with days of backwash period, 18.3NTU in spent, 25.7NTU in reactivated carbon with days, and 40.2NTU in reactivated with days Table The turbidity of discharged water according to changing water position before backwashing The maximum turbidity of discharged water(NTU) Water position L before backwashing Water position LL before backwashing Increase and decrease Spent Spent Reactivated 31.5 48.5 53.2 47.1 66.8 78.9 +15.6 +18.3 +25.7 Reactivated 60.8 101.0 +40.2 Division 4days of back wash 6days of back wash 8days of back wash - 193 - Remarks *Water temp at the time of measurement : 4∼9℃ Journal of Water and Environment Technology, Vol.1, No.2, 2003 Fig shows changes in the turbidity of discharged water from backwashing caused by backwash time and water position change before backwashing All the basins, experimental targets, were the most efficient in backwashing by air for 12 and by water for 20 As a result, when backwash efficiency is evaluated by the turbidity of discharged water, it is judged to be more efficient by controlling water position before backwashing than by controlling backwash time S p e n t c a rb o n ( d a y s o f b a c k w a s h p e rio d ) Turbidity(NTU) 120 100 80 60 40 20 10 15 16 17 W a te r p o s itio n L * 18 19 21 B a c k w a s h tim e ( m in ) 23 W a te r p o s itio n L * 25 27 29 31 29 31 29 31 W a te r p o s itio n L L ** S p e n t c a rb o n ( d a y s o f b a c k w a s h p e rio d ) Turbidity(NTU) 120 100 80 60 40 20 10 15 16 17 W a te r p o s itio n L * 18 19 21 B a c k w a s h tim e ( m in ) 23 W a te r p o s itio n L * 25 27 W a te r p o s itio n L L ** R e a c tiv a te d c a rb o n ( d a y s o f b a c k wa s h p e rio d ) Turbidity(NTU) 120 100 80 60 40 20 10 15 16 17 W a te r p o s itio n L * 18 19 21 B a c k w a s h tim e ( m in ) 23 W a te r p o s itio n L * 25 27 W a te r p o s itio n L L ** R e a c tiv a te d c a rb o n ( d a y s o f b a c k w a s h p e rio d ) Turbidity(NTU) 120 100 80 60 40 20 10 15 16 17 W a te r p o s itio n L * *: of air wash, 18 19 21 B a c k w a s h tim e ( m in ) W a te r p o s itio n L * 18 of backwash **: 12 of air wash Fig.1 The turbidity of discharged changing water position before backwashing water - 194 - 23 25 27 29 31 W a te r p o s itio n L L ** 20 of backwash according to backwash time and Journal of Water and Environment Technology, Vol.1, No.2, 2003 2) The minimum fluidization velocity according to changing water position before backwashing The minimum fluidization velocity, that at the beginning of fluidization, is also the smallest velocity for expanding filter medium2) The point of the minimum fluidization was when air inflow gradually increases loss head and then its difference keeps constant without increasing Table shows the results of examining the minimum fluidization by 12-minute air wash or 20-minute water wash In order to reach the minimum fluidization at the L water position, spent carbon has 6min of air wash and reactivated has 10min of air wash At the LL water position, spent carbon requires 4-minute air wash; reactivated carbon 8-minute air wash Like this, difference in the point of the minimum fluidization between spent and reactivated carbon results from the height of an activated-carbon layer and the condition of activated carbon, etc Also, backwashing at the LL water position required less time for the minimum fluidization than at the L water position; the loss head of both spent and reactivated carbon at the LL water position was 12㎝ higher than at the L water position Generally, backwashing is done by higher than the minimum fluidization velocity; increasing water temperature needs much more increase in backwash velocity because of decreasing water viscosity and lessening attraction between filter media Thus, lowering water position before backwashing, not backwashing by extended time, lessens time for the minimum fluidization, which can raise backwash velocity, strengthen its force over the filter medium of activated carbon, and lessen the loss of activated carbon by backwashing The above results put together, the method of increasing backwash effect is thought to lower water position(LL) before backwashing and to backwash at over the minimum fluidization velocity Reactivated carbon can reach the minimum fluidization by 8-minute air wash; as 18 of water wash goes down to less than 5NTU, 10-minute air wash and 18-minute water wash has turned out to be proper Table The backwashing minimum fluidization according Spent carbon(㎝) Division to changing water position before Reactivated carbon(㎝) Water position L Water position LL Water position L Water position LL before before backwashing before backwashing backwashing before backwashing Air wash 0min 0 0 Air wash 2min 30 44 22 28 Air wash 4min 34 48 30 38 Air wash 6min 36 48 31 41 Air wash 8min 36 48 32 48 Air wash 10min 36 48 36 48 Air wash 12min 36 48 36 48 - 195 - Remarks *Water temp at the time of measurement : 4∼9℃ Journal of Water and Environment Technology, Vol.1, No.2, 2003 The results of an experiment for determining backwash period The test of determining backwash period has compared the changes of water quality factors and tried to find out the right date of backwash and control factors, while operating and not backwashing for 10 days every early season 1) Changes in the water-quality of raw water Table show changes in the water quality of raw water Changes in water temperature are obviously different according to seasons: the average tmeperature of autumn is 13.4℃; that of winter is 3.2℃; that of spring 10.2℃; that of summer 27℃ pH was 7.8∼8.6 on the average, and especially 8∼9 in spring; it's because the dry season can produce a very large quantity of algas And the density of chlorophyll-a has been found the highest as 67.5ppb The average turbidity ranged from to 18NTU as a typhoon, rainfall, and more caused high turbidity Fluctuations in TOC, THMFP, UV254, KMnO4 consumption were comparatively high in spring and summer influenced by the dry season, rainfall, etc.; the consumption of KMnO4 averages 6.3∼9.1㎎/l ; UV254 averages 0.040∼0.068㎝-1; TOC averages 2.74∼3.53㎎/l; THMFP averages 0.0820∼ 0.1388㎎/l Table Changes in the water quality of raw water Division Average Autumn Maximum Minium Average Winter Maximum Minimum Average Spring Maximum Minimum Average Summer Maximum Minimum Water temperatur ( ℃ ) 13.4 18.4 11.0 3.2 5.9 0.8 10.2 13.5 7.2 27.0 30.0 24.8 pH 7.8 8.4 7.3 8.0 8.2 7.6 8.6 9.0 8.1 8.0 8.5 7.1 Turbidity Chlorophyll-a (ppb) (NTU) 33 11 44 13 17 10 18 66 18.6 26.1 10.8 6.1 6.9 4.6 67.5 113 35 21.3 40.1 7.9 KMnO4 comsuption (㎎/l ) 6.5 9.6 5.3 6.3 12.9 4.7 9.1 10.6 7.0 8.4 13.7 5.6 UV254 -1 (㎝ ) TOC (㎎/l ) THMFP (㎎/l ) 0.047 0.068 0.030 0.040 0.067 0.032 0.045 0.056 0.036 0.068 0.094 0.055 2.74 3.22 2.00 3.18 4.06 2.77 3.53 3.80 3.17 3.39 4.16 3.13 0.0890 0.1210 0.0650 0.0820 0.1080 0.0559 0.1388 0.2249 0.1090 0.0878 0.1170 0.0728 2) Changes in the water quality of outflow water from activated-carbon filtration according to days of operation after backwashing (1) Changes in turbidity Fig shows the turbidity of outflow water from spent-, reactivated-, and virgin-carbon filtration, almost the same as or a little lower than that of ozonized water: it's 0.08∼0.13NTU in spring; 0.06 ∼0.10NTU in summer; 0.06∼0.10NTU in autumn; 0.06∼0.11NTU in winter Particularly, turbidity is somewhat higher in spring than in any other season, which results from gradual rise in water temperature and a very large quantity of generation of algas during the dry season; that seems to require much care in waterworks However, considering the above-mentioned results, the - 196 - Journal of Water and Environment Technology, Vol.1, No.2, 2003 turbidity of outflow water from activated carbon-filtration has little change according to days of operation after backwashing; turbidity can't be a factor of operation in determining the date for backwashing Turbidity(NTU) 0.16 0.12 0.08 0.04 0.00 10 Autumn 10 Winter Days R ea ctiv a ted carbo n S pent carbo n Spring Summer Virg in ca rbo n O zo nized water Fig Changes in the turbidity according to days of seasonal operation (2) Changes in organic matter Fig shows changes in organic matter according to days of seasonal operation after backwashing As for spent carbon, the removal rate of KMnO4consumption, UV254, TOC, THMFP, etc has turned out to be just 10% by examination; the low rate has resulted from a falling-off in absorption In autumn, months after the beginning of operation, reactivated carbon has, in removal rate, 50% of KMnO4 consumption, 43% of UV254, 40% of TOC, 27% of THMFP and virgin carbon has, in removal rate, 45% of KMnO4 consumption, 33% of UV254, 39% of TOC, 25% of THMFP; the removal rate of reactivated carbon is higher than that of virgin carbon The longer days of operation, the less removal rate During the 10-day operation after backwashing, the removal rate of organic matter had little difference As a result, the removal efficiency of KMnO4 consumption, TOC, UV254, and THMFP seems to be directly influenced by the quality of flowing-in water and the degree of breakdown of activated carbon more than by days of operation after backwashing; it's improper to see changes in the removal rate of organic matter as a source determining the time for backwashing K M n O c o n s u m p tio n Removal rate(%) 80 40 0 10 Autumn 10 W inter Spring Summer Da ys S pent c arbo n R eac tivated c arbo n Virgin c arbo n Fig Changes in the removal rate of organic matter according to days of seasonal operation - 197 - Journal of Water and Environment Technology, Vol.1, No.2, 2003 UV 254 Removal rate(%) 80 40 0 10 Autumn 10 W inter Spring 8 Summer Da ys S pent c arbo n R eac tivated c arbo n Virgin c arbo n TO C Removal rate(%) 80 40 0 10 Au tu mn 10 W in te r S p rin g S u mme r Da y s S pe nt c a rbo n R e a c tiv a te d c a rbo n Virgin c a rbo n THM FP Removal rate(%) 80 40 0 10 A u tu m n 10 W i n te r S p ri n g Summer Da ys S p e n t c a rb o n R e a c tiv a te d c a rb o n V irg in c a rb o n Fig Changes in the removal rate of organic matter according to days of seasonal operation (3) HPC changes Fig shows the results of HPC according to days of seasonal operation after backwashing; in autumn, spent carbon has 570∼7,400 CFU/㎖; activated has 420∼5,200CFU/㎖; virgin has 380∼ 8,300CFU/㎖ In winter, spent carbon has 2,200∼12,300CFU/㎖ in HPC; reactivated has 1,400 ∼9,400CFU/㎖; virgin has 1,500∼8,700 CFU/㎖ In spring, the dry season, over-generation of algas and activated carbon-attached germs(Table 8) seem to get the number of flowing-out germs to greatly increase: spent carbon has 20,200∼97,500 CFU/㎖; reactivated has 7,400∼90,300CFU/㎖; virgin has 13,600∼95,300CFU/㎖ In summer, in HPC, spent carbon has 3,100∼7,200CFU/㎖; reactivated has 2,400∼8,800CFU/㎖; and virgin has 2,300∼8,400CFU/㎖ The average HPC of flowing-out water from activated carbon-filtration is 1.1∼1.5ⅹ104CFU/㎖, the same as that, 1.5∼ 3.0ⅹ104CFU/㎖, examined by Mr Park et al.,(2001)9), but lower than that by Servails et al.,(1991)12), 4.1ⅹ104∼1.5ⅹ107CFU/㎖ The above experimental results put together, seasonlessly, on the first day after backwashing, HPC increases and then gradually decreases; the longer days of operation, the higher HPC That seems to have some relation to the growth of - 198 - Journal of Water and Environment Technology, Vol.1, No.2, 2003 microorganism in the absorption of activated carbon; therefore, prevention of flowing-out microorganisms by their over-propagation requires periodical backwashing based on observation of HPC As a result, in autumn as microorganisms begin to increase on the 6th day of operation, autumn needs backwashing on the 5th or 6th day; winter needs backwashing on the 4th or 6th day of operation In spring, HPC begins to increase on the 4th day, but spring has much more flowing-out germs, so backwashing should be done on the 2nd or 4th day Summer has a little fewer flowing-out germs; however, high turbidity of water flows in because of typhoon or the rainy season, which results in rise in water temperature and the density of organic matter Considering these factors, summer requires backwashing on the 3rd or 5th day of operation On the other hand, spent carbon has a little more flowing-out germs than reactivated or virgin carbon does; it's desirable to make the backwashing date in spent carbon earlier than in activated or virgin carbon HPC(CFU/ml) E + E + E + E + 2 10 A utum n 10 W in t e r S p rin g S um m er D a ys S p e n t c a rb o n R e a c tiva te d c a rb o n Virg in c a rb o n Fig Changes in HPC according to the days of seasonal operation after backwashing (4) The number of activated carbon-attached germs The number of granular activated carbon-attached germs has been found as 1.4ⅹ105∼5.8ⅹ 108 CFU/g in case of spent carbon, 1.1ⅹ105∼1.6ⅹ108 CFU/g in case of reactivated carbon, and 6.6ⅹ105∼5.8ⅹ108 CFU/g in virgin carbon The number with these three kinds of activated carbon was comparatively high in November, March to April, and September; decreases from December when water temperature began to fall; is the lowest in January to February and a little low in July to August with over 30℃ of water temperature(Table 8, Fig 5) This is higher than the number by Mr Kim(1995)13), 106∼107CFU/g, and similar to that by Mr Park et al.,(2001)9), 0.6∼9.8ⅹ108CFU/g The number of germs attached to reactivated or virgin carbon usually remains over 106CFU/g from the 60th day of operation on, so its granular carbon contact basin was found to be managed by BAC; the number of granular activated carbon-attached germs has revealed that free residual chlorine or residual ozone doesn't influence BAC process very much In spring with increasing granular activated carbon-attached germs, HPC flowing into the filter water of granular activated carbon also increases; therefore, it's necessary to advance the date of backwashing, to observe if backwashing is going well, and to take care of the existing process including a sand filter basin - 199 - Journal of Water and Environment Technology, Vol.1, No.2, 2003 Table The monthly number of activated carbon-attached germs The number of attached germs (CFU/㎖) Spent carbon Reactivated carbon Virgin carbon 7.3 106 2.7 106 5.4 106 8 1.2 10 1.6 10 3.2 108 7 3.7 10 5.3 10 1.0 107 2.0 106 4.5 106 1.3 107 1.4 10 1.1 10 6.6 105 5.8 108 1.1 108 5.8 108 5.6 10 1.4 10 5.6 107 6 6.4 10 5.3 10 3.1 106 1.1 107 2.9 106 2.1 107 8 1.0 10 1.1 10 5.0 107 1.8 107 1.3 107 3.8 107 Division 2001 10 2001 11 2001 12 2002 2002 2002 2002 2002 2002 2002 2002 10 HPC(CFU/g) E + E + E + E + E + E + 10 11 12 10 Month S pent c arb on R eac tivated c arbon V irg in c arb on Fig Monthly changes in the number of activated carbon-attached germs Ⅳ Conclusion For the efficient operation of granular-activated-carbon absorption process in advanced water purification, the study had performed a efficiency test of backwashing and a backwash-period-determining test, with granular activated carbon(spent, reactivated, and virgin carbon) basins as the subjects of study, among 24 ones in the M water purification plant in Daegu metropolitan city, from October 2001 to September 2002 And its results are as following; An efficiency experiment according to backwash methods 1) The turbidity of flowing-out water according to changes in backwashing has proved to be 4.2∼ 17.3NTU higher in 8-minute air wash or 18-minute water wash than 10-minute air wash or 20-minute water wash 2) The turbidity of flowing-out water according to changes in water position before backwashing has turned out to be 15.6∼40.2NTU higher at water position LL before backwashing than at water position L - 200 - Journal of Water and Environment Technology, Vol.1, No.2, 2003 3) The point of the minimum fluidization differed according to kinds of activated carbon like spent or reactivated carbon: at the L water position before backwashing, spent carbon(the 16th basin) required of air wash, and reactivated carbon(the 19th) 10 of air wash; at the LL, spent carbon(the 16th) required of air wash, and reactivated carbon(the 19th) 18 of air wash 4) Backwashing should be done at over the minimum fluidization velocity, but changing water position could raise the efficiency of backwashing more than controlling backwash time; the LL water position before backwashing had 10-minute air wash and 18-minute water wash as the most proper condition A test determining backwash period 1) The turbidity of outflow water from activated-carbon filtration and changes in organic matter had no big difference according to days of seasonal operation after backwashing 2) Changes in HPC according to days of operation after backwashing was a little high on the first day after backwashing, and decreased gradually; the more days of operation, the higher HPC Particularly, HPC was the highest in spring 3) The number of granular activated carbon-attached germs was 1.1ⅹ105∼5.8ⅹ108CFU/g, and granular activated carbon proved to be operated by BAC The number of spent carbon-attached, reactivated carbon-attached, and virgin carbon-attached germs were high in November, March, and September; the lowest in February; especially high from March to April 4) The date of backwashing was closely connected with the growth of microorganism, and greatly influenced by changes in water quality; therefore, HPC range, the condition of water quality, the condition of a filter basin, and the years of activated-carbon use should be considered for operation References Carlson, M.A et al., "Comparing two GACs for Adsorption and Biostabilization", J AWWA, 86(3): 91-102 1994 Cho, W.H et al., A Study on the Optimization of Activated-carbon Backwashing Water technology Research Institute of Seoul Metropolitan Government in Korea, 219-275 1999 The Ministry of Environment, Development of Automation and Related Facilities for Water System Using Ozone and Activated Carbon Korea, 858-884 2001 The Ministry of Environment, The Official Test Method of Drinking-Water Quality Korea, 1999 - 201 - Journal of Water and Environment Technology, Vol.1, No.2, 2003 APHA, AWWA and WPCF, Standard Methods of the Examination of Water and Wastewater 17th ed., Washington, D.C., USA., 1987 The Japanese Waterworks Society, The Japanese Test Method of Waterworks 1985 Rice, RG and C.M Robson., Biological Activated Carbon Lewis publishers, Boca raton, Florida 1982 Nagazawa, The Movement of Microorganism in a Granular Activated Carbon Layer The 41st Japanese waterworks research announcement meeting 1990 Park, HG et al., Improving Water Quality and Bacterial Characteristics during water Treatment Process Using Biologica Activated Carbons on Downstream of the Nakdond River J Korean Environmental Sciences, 10(2): 105-111 2001 10 Kim, B.G., A Study on the Efficiency of Backwashing in the Adoption Process of Activated Carbon Kyeongnam university graduate school 2001 11 Yu, P.J., A Study on the Optimum Backwash Velocity by Water Temperature Change Seoul industrial university graduate school 2000 12 Servais, P., G Billen, C Ventresque and G.P Bablon., Microbial activity in GAC filters at the Choisy-Roi treatment plant J AWWA 75: 62-68 1991 13 Kim, J.Y., A Study on the Intensive Waterworks of Raw Water by Activated Carbon., Dohoku university, Japan 1995 14 Park, J.H., Drinking Water Microbiology Chemical Engineering Corporation, Korea 109-110 1990 - 202 - ... 2003 The characteristics of a granular activated carbon In a granular activated carbon contact basin, virgin carbon is domestic activated carbon made from palm shell, reactivated carbon means activated. .. conditions and methods The study selected basins(2 reactivated, spent carbon basins, and virgin carbon basin) of granular activated carbon from 24 ones in the M water purification plant in Daegu;... carbon 24 granular activated carbon contact basins consist of buildings each of which has stationary downward filter basins The rate of activated carbon and sand is 250:20(㎝); the under drainage